1. Field of the Invention
The present invention relates to a light emitting device comprising a plurality of pixels each of which includes a light emitting element and a means for supplying current to the light emitting element.
2. Description of the Related Art
A light emitting element can exhibit highly visible light as it is a self-luminous type. Also, it is suitable for slimming of a display device and can provide a wide viewing angle differently from liquid crystal display devices (LCDs) which require backlight. Therefore, a light emitting device formed by using light emitting elements is attracting attentions as a substitute for CRTs and LCDs, and is now being applied to various electronic devices such as mobile phones and digital still cameras. Among them, an active matrix light emitting device which is actively developed in recent years typically comprises pixels each of which includes at least a light emitting element, a TFT for controlling an input of a video signal to the pixel (switching TFT) and a TFT for controlling a current value supplied to the light emitting element (driving TFT) although the configuration of the pixel differs among manufacturers.
A TFT (Thin Film Transistor) formed by using a polycrystalline semiconductor film has a mobility of 100 or more times higher than a TFT formed by using an amorphous semiconductor film, and it has the advantage that when constructing a light emitting device, a pixel portion and its peripheral driver circuit can be formed integrally on the same substrate. The polycrystalline semiconductor film can be formed over an inexpensive glass substrate by using laser annealing. However, energy of laser light outputted from an oscillator has flickers of at least about several % due to various factors, which prevent uniform crystallization of the semiconductor film. When the uniform crystallization is disturbed, which in turn causes variations in crystallinity of a polycrystalline semiconductor film, characteristics (for example, on-current, mobility, threshold voltage and the like) of a TFT using the polycrystalline semiconductor film as its active layer vary. Accordingly, when a polycrystalline semiconductor film formed by laser annealing is used as a driving transistor, characteristic variations of the driving transistor cause luminance variations of a light emitting element.
When a circuit for compensating characteristic variations of the driving transistor is provided in each pixel, luminance variations of a light emitting element resulting from such characteristic variations can be suppressed. However, since the number of TFTs in the pixel is disadvantageously increased when adopting such a method, high resolution cannot be achieved.
In addition, there is another problem that luminance of a light emitting element decays in accordance with the degradation of electroluminescent materials used for the light emitting element. In this case, luminance decay of the light emitting element can be suppressed by supplying constant current to the light emitting element more efficiently than by supplying constant voltage to the light emitting element. However, even when the current supplied to the light emitting element is constant, luminance thereof decays in accordance with the degradation of the electroluminescent materials. The degree of degradation depends on the time that the light emitting element emits light or the amount of current flowing to the light emitting element. Therefore, when a gray scale of each pixel differs according to a displayed image, degradation of the light emitting element in each pixel differs accordingly, which causes luminance variations.
In such a case, the luminance decay due to the degradation of the electroluminescent materials can be suppressed to a certain degree by operating the driving TFT in the saturation region. However, since a value of a drain current of a TFT which operates in the saturation region easily fluctuates by a slight fluctuation of the gate-source voltage (hereinafter referred to as gate voltage) Vgs, luminance of the light emitting element easily fluctuates accordingly. Therefore, when operating the driving TFT in the saturation region, it is required that the gate voltage Vgs thereof do not fluctuate while the light emitting element emits light. However, when off-current of a switching TFT is large, the gate voltage Vgs of the driving TFT easily fluctuates in accordance with the potential change of a video signal which is inputted to other pixels. In order to prevent such fluctuation of the gate voltage Vgs, it is required that a capacitor for holding the gate voltage of the TFT have a high capacity or the off-current of the switching TFT be suppressed low. However, in order to optimize the manufacturing steps of TFTs so as to suppress the off-current of the switching TFT low while increasing the on-current so as to charge a large capacitance in a predetermined period, high cost and long time are required disadvantageously. In addition, an increase of the area occupied by the capacitor is undesirable since it will produce a leak current between opposite electrodes of the capacitor due to dust and the like more often, which leads to lower the yield. Further, there is still another problem that the gate voltage Vgs of the driving TFT easily fluctuates along with the switching operation of other TFTs, a potential fluctuation of a signal line or a scan line and the like due to the parasitic capacitance of the gate electrode.